Method of producing an encapsulated core assembly



C. E. MOYERS Dec. 29, 1964 MET United States Patent 3,1fi2,934 METHOD OF PRODUCENG AN ENCAPSULATED CORE ASEMBLY Chester E. Moyers, Williamsvilie, 111., assignor to Sangamo'Electric (10., Springfield, ill, a corporation of Delaware Filed Jan. 18, 1961, Ser. No. 83,533 Claims. (Cl. 29-15557) This invention is directed to a core assembly for an instrument transformer or inductor, and more particularly to such a core assembly which includes a novel resilient sealing member for protecting the core structure during the final manufacturing steps.

In a current measuring instrument transformer for use in a power distribution system, a core assembly which resembles a hollow cylinder is frequently used. The major portion of the core assembly can be provided of metal, for example, by rolling or winding a rectangular sheet of metal to form a generally cylindrical piece. Sufiicient turns or convolutions are provided to impart rigidity to the core structure, and different metals can also be used in a single core assembly. For example, in one such core assembly Mumetal is utilized to provide the convolutions on the inner one-third of the structure and silicon steel is used as the core material in the outer twothirds of the hollow cylinder. When the core assembly is completely fabricated, the primary conductor can be passed through the bore or window defined in the interior thereof and the secondary winding wound around and through the core assembly to provide a transformer structure. After the conductors are added the core assembly is usually coated with some material such as epoxy resin to both fix the core assembly members in place and protect the structure from adverse weather conditions, mechanical damage, etc.

Certain problems have been encountered during the encapsulation of the core assembly with epoxy resin or related materials. In the first place, it has been noticed that the magnetic characteristics of the core assembly are damaged if the resin penetrates the convolutions of the wound metallic core, which may be caused by adhesion of the resin to the metal surface areas. This problem is aggravated when the epoxy is cast under vacuum. The core assembly is also adversely afiected by the pressure exerted thereon as the epoxy encapsulation contracts during the setting up process. Further, after the outer epoxy coating has completely set up, the coefficient of thermal expansion of the epoxy encapsulation is different than the coefiicient of thermal expansion of the metal core, and thus the epoxy material can exert pressure on the core as a function of ambient temperature even after the encapsulation has been completed.

- It is therefore a primary object of the present invention to provide a sealing member for a core assembly to prevent the entrance or adherence of the epoxy material during the encapsulation of the assembly.

It is another object of the invention to provide such a sealing member which is resilient and thus absorbs at least a portion of the compressive pressures developed during the setting up and subsequent expansion by heating of the epoxy encapsulation.

The foregoing and other objects of the invention are achieved in a core assembly to which is added a novel resilient sealing member, which may be a tube of rubber or a similar material. The tube includes a first end portion, a central portion, and a second end portion. Ini tially, the tube is positioned with its central portion within the window of the core assembly, and the first end portion is then folded back on itself and stretched outwardly to cover a part of the outer surface of the hollow cylindrical core. The second end portion is then folded back upon itself and stretched outwardly to cover the remain der of the core assembly to overlap at least part of the first end portion. The resultant core assembly is both effectively sealed by the action of the resilient tube member, and the tube member likewise absorbs at least a portion of the compressive pressures developed as the epoxy sets up during encapsulation process, as well as the pressures subsequently developed in use during heating of the core assembly and of the epoxy shield.

Now in order to acquaint those skilled in the art with the manner of constructing and utilizing the invention, a preferred embodiment will be explained and described in connection with the accompanying drawing, in the several figures of which like reference numerals identify like elements and in which:

FIGURE 1 is a perspective illustration of a transformer including a core assembly and primary and secondary conductors;

FIGURE 2 is a perspective illustration of a core assembly prior to the addition of the novel sea-ling and resilient member; and

FIGURES 3-5 are perspective illustrations particularly iilustrating the addition of the novel resilient sealing member to a core assembly to protect the same during and after encapsulation.

In the simplified showing of FIGURE 1, transformer 16 is formed to define a bore or window area therein. A primary conductor 12 is passed through this window and terminals 13 and 13 are provided to afford connection to the secondary conductor which is wound around and through the core assembly. The configuration of the core assembly-is shown in FIGURE 2.

As there shown, prior to encapsulation the core assembly comprises a plurality of convolutions of core mate-- rial 15, only the inner one of which is visible in the drawing. As described above, subsequent convolutions can be comprised of different material to achieve any desired magnetic characteristics. To assist in absorbing the compressional forces during the setting up of the epoxy encapsulation and subsequently as the metal core material and the epoxy coating are expanded at difierent rates during heating thereof, a cylindrical resilient member 16 which may be of cork neoprene or other suitable resilient or cushioning material is placed around the outer surface of the core assembly and secured by a short piece of tape 17. It is also desirable to provide a pair of resilient end members 18 and Ztl, which may be of annular-shape and secured in place by the tape segments 21, 22 and 23 as illustrated. The annular cushions or end members may be simple fishpaper washers to prevent the tearing of the novel resilient sealing member as pressures are exerted by the application of the secondary winding to the outer surface of the assembly.

In accordance with the inventive teaching, as shown in FIGURE 3, a resilient sealing member such as a tube section 25 is inserted through the window of core assembly 11. The intermediate portion of sealing member 25 is not visible, being centered within the core assembly, with a first end portion 26 protruding from one end and a second end portion 27 protruding from the other end. As shown in FIGURE 4, the first end portion 26 is then folded back upon itself and stretched outwardly to enclose one end of core assembly 11. The other end 27 is similarly turned or folded back upon itself and stretched outwardly, as shown in FIGURES, to enclose the remainder of the core assembly and overlap by at least a slight amount the extremity of end portion 26. In this way the core assembly 11 is both effectively sealed. the resilient sealing member 25 covering both the in terior, end portions and exterior of the core assembly, and likewise the resilient qualities of the sealing mem ber when formed of rubber or similar material are such as to absorb at least a portion of the forces developed during the epoxy encapsulation and subsequent stressing of the core assembly when the resultant enclosed structure is heated. The cylindrical resilient or" cushioning member 16 within the core assembly absorbs any part of the radial compressional forces which might be too great for complete absorption by the resilient sealing member 25. V

In a preferred embodiment, a core assembly having an inner diameter of 1.5 inches, an outer diameter of 2.5

inches, and a length of 2.25 inches, was suitably sealed with a six inch section of rubbe'r tubing; The material was sufficiently taut when stretched thercover as indi cated FIGURE 5 to provide a smooth-fitting cover. However, even if the covering is not perfectly smooth, the same eifect can be achieved by subjecting the enclosed core assembly to a vacuum prior to encapsulation. The sealing member 25 may beiof rubber, neoprene, or any other suitably elastic material, the elasticity being required to permit folding the member back upon itself and working it over the outer portion of the core assembly prior to encapsulation. Those skilled in the art will recognize that difierent materials can be utilized to eifect this end, and accordingly applicant has utilized the term resilient sealing member to describe a suitable tubedilre sealing element both in the specification and in the appended claims.

While only a particular embodiment of the invention has been illustrated and described, it is apparent that alterations and modifications may be made therein, and it is intended in the appended claims to cover all such alterations and modifications as may fall within the true spirit and scope of the invention.

What is claimed is:

1. The method of producing an encapsulated core assembly for an instrument transformer wherein a straight length of the primary conductor passes through the core assembly to function as the primary element of the instrument transformer, said method comprising the following steps: shaping a metallic core assembly in the form of a hollow cylinder having a central bore therethrough, through which bore the straight length of primary conductor is adapted to pass, inserting a tubular elastic sealing member through said central bore, said tubular elastic sealing member having a normal length in excess of twice the length of said core assembly and being placed in said core assembly to have its opposite end portions initially project approximately equal distances beyond the opposite ends of said core assembly folding one of said end portions back upon itself and stretching said one end portion elastically outwardly to cover one portion of said core assembly, folding the other of said end portions back upon itself and stretching said other end portionelastically outwardly to cover the remainder of said core assembly, causing said other end portion to overlap at least part of said one end portion, and thereafter encapsulating the enclosed core assembly to provide a reenforced unit, the elasticity of said tubular sealing member causing the first and second end portions to contract inwardly for permanently establishing a snug elastic contact directly with the external and internal surfaces of said core assembly so as to prevent the encapsulating material from contacting said metallic 7 4 having a diameter closely approximating the diameter of said central bore and having a normal length in excess of twice the length of said core assembly, and being placed in said core assembly to have its opposite end portions initially project approximately equal distances beyond the opposite ends of said core assembly folding one end portion of said tubular sealing member elastically 'out- 7 wardly to cover one end portion of said core assembly, folding the other end portion of said tubular sealing member elastically outwardly to cover the other end portion of said core assembly, stretching the latter end portion outwardly to overlap the end of said first end portion, mounting electrical conductor means on said core assembly, and then encapsulating the core assembly, said tubular elastic sealing member preventing the encapsulating material from contacting said magnetic core assembly in theencapsulat-ing operation.

3. The method of constructing a transformer core I which is adapted to be encapsulated with an epoxy resin or like material, which comprises shaping a magnetic core assembly to have a central passageway extending therethrough from end to end, inserting a tubular elastic sealing member through said central passageway, said tubular elastic sealing member having anormal length slightly in excess of twice the axial length of said core assembly, and said sealing member being placed in said core assembly to have its opposite end portions initially project substantial distances in opposite directions beyond the opposite ends of said core assembly, folding one end portion of said tubular sealing member elastically outwardly to cover one end portion of said; magnetic core assembly, folding the other end portion of said tubular sealing member elastically outwardly to cover the other end portion of said magnetic core assembl causing one of said end portions to overlap and envelopthe extremity of. the other end portion to establish a hermetically seal' ing enclosure of saidlmagne ic core assembly, mounting electrical conductor means on said magnetic core assembiy, and then encapsulating the core assembly, said tubular sealing member preventing the encapsulating material from contacting or entering into said magnetic core assembly in the encapsulating operation.

4. The method of producing an instrument transformer oi the toroidal encapsulated type having an axial opening therethrough for assembling over'a single straight length of high voltage conductor which is adapted to pass straight through said axial opening to function as the primary conductor for the instrument transformer, which method comprises:winding a relatively wide thin strip of magnetic core metal in spiral form through a plurality of successive spiral convolutions to form a cylindrical core structure having the aforesaid axial core opening therethrough and having a cylindrical length greater than the diameterof said axial opening, securing the spirally wound turns of said core structure against unwinding, passing an impervious tubular boot composed of thin highly elastic rubber through said axial opening, said tubular boot being normally of substantially uniform diameter from end to end with an outside diameter closely approximating the inside diameter of said axial open ing, said elastic boot being inserted into said core struc: cure to have right and left hand projecting ends thereof extending beyond the right and left hand ends of said cylindrical core structure to distances which ar'egreater than the radial dimensions of the ends of said cylindrical core structure, taking the right hand projecting end of said elastic bootand stretching it radially outwardly ture, performing substantially the same operation with said left band'projecting end to extend it outwardly across the left hand end of said cylindrical core structure and then doubling it back toward the right over the cylindrical surface of the .core structure so as to elastically envelop the left hand portion of said core structure, lapping said doubled back left hand end over the doubled back right hand end of said elastic boot so as to form a complete sealing enclosure which imperviously envelops the entire core structure including the cylindrical outer surface, the cylindrical inner surface and the radial end faces thereof, then winding a secondary winding over said core structure and said enclosing elastic boot, with the turns passing through said axial opening, and thereupon encapsulating the resulting unit in an epoxy compound, said impervious boot preventing the mass of epoxy compound from penetrating into the spaces between the spiral convolutions of said core structure.

5. The method recited in claim 4 wherein a layer of cushioning material is placed around said cylindrical core structure before said impervious elastic boot has its ends doubled back over said core structure.

References Cited in the file of this patent UNITED STATES PATENTS 1,485,289 Peterson Feb. 26, 1924 2,118,924 Henderson May 31, 1938 2,230,067 Pedlow Jan. 28, 1941 2,444,737 Heath July 6, 1948 2,519,495 Nesbitt et a1 Aug. 22, 1950 2,592,721 Mott Apr. 15, 1952 2,648,124 Duyck Aug. 11, 1953 2,910,814 Yelinek Nov. 3, 1959 2,920,297 Spicer Jan. 5, 1960 2,947,959 Polzella et a1. Aug. 2, 1960 3,005,542 Harrison Oct. 24, 1961 

1. THE METHOD OF PRODUCING AN ENCAPSULATED CORE ASSEMBLY FOR AN INSTRUMENT TRANSFORMER WHEREIN A STRAIGHT LENGTH OF THE PRIMARY CONDUCTOR PASSES THROUGH THE CORE ASSEMBLY TO FUNCTION AS THE PRIMARY ELEMENT OF THE INSTRUMENT TRANSFORMER, SAID METHOD COMPRISING THE FOLLOWING STEPS: SHAPING A METALLIC CORE ASSEMBLY IN THE FORM OF A HOLLOW CYLINDER HAVING A CENTRAL BORE THERETHROUGH, THROUGH WHICH BORE THE STRAIGHT LENGTH OF PRIMARY CONDUCTOR IS ADAPTED TO PASS, INSERTING A TUBULAR ELASTIC SEALING MEMBER THROUGH SAID CENTRAL BORE, SAID TUBULAR ELASTIC SEALING MEMBER HAVING A NORMAL LENGTH IN EXCESS OF TWICE THE LENGTH OF SAID CORE ASSEMBLY AND BEING PLACED IN SAID CORE ASSEMBLY TO HAVE ITS OPPOSITE END PORTIONS INITIALLY PROJECT APPROXIMATELY EQUAL DISTANCES BEYOND THE OPPOSITE ENDS OF SAID CORE ASSEMBLY FOLDING ONE OF SAID END PORTIONS BACK UPON ITSELF AND STRETCHING SAID ONE END PORTION ELASTICALLY OUTWARDLY TO COVER ONE PORTION OF SAID CORE ASSEMBLY, FOLDING THE OTHER OF SAID END PORTIONS BACK UPON ITSELF AND STRETCHING SAID OTHER END PORTION ELASTICALLY OUTWARDLY TO COVER THE REMAINDER OF SAID CORE ASSEMBLY, CAUSING SAID OTHER END PORTION TO OVERLAP AT LEAST PART OF SAID ONE END PORTION, AND THEREAFTER ENCAPSULATING THE ENCLOSED CORE ASSEMBLY TO PROVIDE A REENFORCED UNIT, THE ELASTICITY OF SAID TUBULAR SEALING MEMBER CAUSING THE FIRST AND SECOND END PORTIONS TO CONTRACT INWARDLY FOR PERMANENTLY ESTABLISHING A SNUG ELASTIC CONTACT DIRECTLY WITH THE EXTERNAL AND INTERNAL SURFACES OF SAID CORE ASSEMBLY SO AS TO PREVENT THE ENCAPSULATING MATERIAL FROM CONTACTING SAID METALLIC CORE ASSEMBLY, SAID ELASTIC SEALING MEMBER ALSO FORMING A LAYER OF ELASTIC CUSHIONING MATERIAL OVER SAID EXTERNAL AND INTERNAL SURFACES OF THE CORE ASSEMBLY TO ELASTICALLY ABSORB SOME OF THE COMPRESSION PRESSURES ARISING DURING THE ENCAPSULATING OPERATION. 